Micro gas chromatography (μGC) is based on developing miniaturized, portable systems capable of identifying the composition of a gas mixture by separation into its individual components and are applicable for homeland security, space exploration, on-site or distributed environmental monitoring mechanisms, and food assessment. In a typical μGC system, the sample mixture is first collected on an adsorbent bed referred to as the pre-concentrator. When thermally spiked, this device releases the adsorbed species in a sharp vaporized plug. This narrow plug enters a microfluidic channel, called the separation column, which is coated with a stationary phase film to chemically interact and retard the various analytes of the plug to different extents. The analytes are then separated in time and, ideally, elute out of the column one-by-one into a detector. An inert carrier gas (mobile phase) such as helium or nitrogen facilitates the movement of the analytes through the entire system.
Miniaturization offers unique advantages such as lightweight, low power consumption, less reagent usage and innovative architectures apart from lower cost when batch fabricated. Stereotypical miniaturization utilizes components fabricated in silicon/glass. Common implementations involve etching a narrow bore microfluidic channel in silicon/glass wafers, with capillary dimensions similar to conventional GC columns, or fabricating posts within the silicon cavity and coating with an adsorbent material. The primary incentive is the ability to conveniently pack a 1-2 m length tubing (cavity) into a 2 cm×4 cm×500 μm silicon die without having to wind equivalent length capillary tubing into a large coil. In addition, heating a silicon die with on-chip heaters is energetically far less taxing compared to heating capillary tubing with a convection oven.
The choices for detectors in the micro-world are numerous. While traditional GC systems are dominated by flame ionization detectors (FID), electron capture detectors (ECD) and flame photometric detectors (FPD), μGC offers the possibility of obtaining signals via other forms of reactive processes using sorptive sensors that transduce into electrical, acoustic or optical domains. In general, any concentration-sensitive detector, such as the thermal conductivity detector (TCD), are more pliable to be reduced in size. It should be noted that while ionization detectors such as the FID provide robust performance and sensitivity, efforts to miniaturize them do not yield comparable detection levels since the hydrogen flame loses its ionizing potential when reduced in size. On the other hand, sorptive and thermal sensing detectors have inherent limitations since they are more temperature sensitive and hence their implementation and application has been inadequate as well. Mass spectrometry (MS), considered the gold standard in conventional analytical techniques, has also been subject to miniaturization. A majority of these efforts have focused on reducing the size of a MS using techniques that are not found in silicon micromachining. This has resulted in dimensions slightly larger than that found in μGC and a power dissipation on the order of tens of watts.
Commercially available μGC systems have adopted a hybrid approach wherein the detector is similar in style to conventional ultraviolet photoionization detectors (UV-PID). These systems offer excellent detection sensitivity, but are somewhat restricted by the photoionization energies available (<11.7 eV with argon lamps) as well as incorporation into a μGC system.
Micro-discharges or plasmas have also been utilized in gas detectors. One such detector uses fragmented analytes in a DC microplasma to produce diatomic fragments from which emission are detected spectrophotometrically. Improvements on this technique included an innovative electrode structure to generate a pulsed plasma with drastically reduced power consumption.
However, spectrophotometric detection is an intensive operation that consumes power on the order of watts. An alternative is to monitor the current through the discharge itself. However, a common concern with these designs is the fouling of the electrodes due to fragmentation of the analytes. Fragmentation also does not allow for the analytes to be subjected to further analysis.